The present invention relates to an ultrasonic imaging apparatus for forming a tomogram of a subject by applying ultrasonic beams to the subject and then by processing the echo waves emitted from the subject.
An ultrasonic imaging apparatus can perform linear scanning or sector scanning on a subject by using ultrasonic beams, receive the echo waves emanating from the points where the beams have been reflected, and process the echo signals obtained from the echo waves, to thereby form a tomogram of the region of interest. The apparatus has a linear array of transducer elements, and a delay circuit including delay elements coupled to the transducer elements, respectively. The echo signals are processed by the phased array method. The delay elements are indispensable for the phased array method. The delay elements have different delay times. The delay time of each delay element is the difference between the period of time which an echo wave requires to travel from the region of interest to the transducer element coupled to the delay element and the period of time which an echo signal requires to travel from the region of interest to the next transducer element. Each transducer element converts the echo signal into an electric echo signal, and supplies this signal to the corresponding delay element. The delay element delays the signal by the delay time. As a result, the delay elements provide echo signals in the same way as in the case where the echo waves are in phase when they reach the respective transducer elements.
The delay elements, which play an important role in the phased array method, are so-called LC delay lines each comprised of an inductance and a capacitance. The LC delay line has several taps. One of the taps is selected, thereby providing a desired delay time by which an echo signal will be delayed.
The LC delay line is expensive, however. This raises the cost of the delay circuit. The LC delay line is also disadvantageous in that its delay time cannot be minutely adjusted. Further, it is possible that the LC delay lines fail to have the desired delay times since the inductances and capacitances have different characteristics from one another. Another problem with the LC delay line is that noise is generated as one tap is switched to another, and this noise will inevitably enter the echo signal (an analog signal) input to the delay circuit. For instance, in order to accomplish dynamic focussing, that is, to move the point at which the ultrasonic beam is focussed, while the echo wave corresponding to this beam is being received by a transducer element, one tap must be switched to another. This tap-switching generates noise, which will unavoidably mingle with the echo signal output from the transducer element.